In the mobile radio communication field which is markedly spreading in these years, for example, a frequency band of 70 to 400 MHz is employed as the intermediate frequency of communication equipment. Bandpass filters known in the prior art for use in such a frequency band of more than 50 MHz include LC filters, monolithic quartz crystal filters (sometimes abbreviated as MCF) utilizing piezoelectric bulk wave, and surface acoustic wave (sometimes abbreviated as SAW) filters. Among others, multi-mode type surface acoustic wave filters are often used in the recent years.
The multi-mode type surface acoustic wave filters operate on a principle which is very similar to the well-known operating principle of MCF. The filter includes a plurality of juxtaposed resonators each including a piezoelectric substrate and an interdigital transducer (abbreviated as IDT, hereinafter) formed thereon wherein excitation of dominant and higher modes occurs by acoustic coupling. A pattern design is made such that the anti-resonance frequency of the dominant mode may coincide with the resonance frequency of the higher modes.
The multi-mode type surface acoustic wave filters include transversely coupled multi-mode filters and longitudinally coupled multi-mode filters. The transversely coupled multi-mode filters utilize a displacement distribution of a primary mode (referred to as a symmetric mode, hereinafter) and a secondary mode (referred to as an antisymmetric mode, hereinafter) existing in a direction perpendicular to the propagation direction or transverse direction as shown in FIG. 1(a). The longitudinally coupled multi-mode filters utilize a displacement distribution of a symmetric mode and an antisymmetric mode existing in the propagation direction or longitudinal direction as shown in FIG. 1(b). Usually a plurality of such multi-mode type surface acoustic wave filters are connected in cascade in order to improve an out-of-band attenuation. In general, both transversely and longitudinally coupled multi-mode filters include reflectors at opposite sides of the element for the purpose of increasing the Q value. Since both the transversely and longitudinally coupled multi-mode filters operate on the same principle, only the transversely coupled multi-mode filter is described in further detail for avoiding redundancy.
The multi-mode type surface acoustic wave filter has a pass-band width which is generally represented by a fractional bandwidth (.DELTA.f/f), that is, the frequency difference .DELTA.f between the resonance frequency frs of a symmetric mode and the resonance frequency fra of an anti-symmetric mode divided by the center frequency f. The frequency difference .DELTA.f (=fra-frs) between the two modes has a certain value, and .DELTA.f may be adjusted by the design of IDT electrodes or patterning of the reflectors, however, its upper and lower limits are determined by an electromechanical coupling factor K.sup.2 of a piezoelectric substrate material.
Quartz crystal is often used as the piezoelectric substrate material. It has a factor K.sup.2 of 0.14 to 0.16% . Then the fractional bandwidth (.DELTA.f/f) of conventional multi-mode type surface acoustic wave filters is limited to about 0.01 to 0.06% as shown in FIG. 8 of Japanese Patent Application Kokai (JP-A) No. 131213/1984. FIGS. 2(a), 2(b) and 2(c) are diagrams illustrating the filter characteristics, impedance characteristics of symmetric and antisymmetric modes, and Smith chart of a transversely coupled multi-mode filter having a fractional bandwidth (.DELTA.f/f) of 0.05%.
It is understood that the fractional bandwidth (.DELTA.f/f) of conventional multi-mode type surface acoustic wave filters has the limit of 0.06%. An attempt to forcibly expand the bandwidth of a multi-mode type surface acoustic wave filter beyond the capability of its substrate material, for example to increase the fractional bandwidth (.DELTA.f/f) to 0.1%, will result in filter characteristics as shown in FIG. 3(a) wherein a large ripple appears in the pass band, which is no longer regarded as a filter. The reason is given below. The difference between the anti-resonance frequency of a symmetric mode and the resonance frequency of an anti-symmetric mode is increased by forcible expansion of the bandwidth beyond the capability of substrate material as seen from the impedance characteristics of symmetric and antisymmetric modes shown in FIG. 3(b). Then the operating impedance of a filter within the pass-band becomes capacitive as shown in the Smith chart of FIG. 3(c), failing to achieve impedance matching with an external circuit and impedance matching between adjoining filters when a plurality of filters are connected in cascade, resulting in an increased insertion loss.
To overcome the above-mentioned problem associated with the expansion of the pass-band width of multi-mode type surface acoustic wave filters, a typical prior art approach uses a matching circuit for achieving impedance matching with an external circuit as shown in FIGS. 6 and 8. More particularly, matching circuits having sufficient inductances La, Lb and Lc to cancel the capacitive reactance of multi-mode type surface acoustic wave filters 1 are added to the connections between a plurality of multi-mode type surface acoustic wave filters 1 connected in cascade and the input and output terminals of multi-mode type surface acoustic wave filters. Actually, since the value of inductance L is several hundred nH in a frequency band of higher than 200 MHz and fine adjustment of the L value is difficult, it is a common practice to combine the inductance La given as a coil with a capacitor C to co-operatively cancel the capacitive reactance of multi-mode type surface acoustic wave filters.
In this case, however, a problem arises with respect to electrical characteristics. The above-mentioned impedance matching method ensures impedance matching in the pass-band, but from the aspect of filter characteristics in a wider band, the impedance matching behaves just like enhanced LC resonance so that the response of LC resonance is predominant among out-of-band characteristics, resulting in a deteriorated out-of-band attenuation. One countermeasure is to add resistance R parallel to L and C as shown in FIGS. 6 and 8 (a) to reduce the Q of LC resonance to increase the out-of-band attenuation, but is not so effective. In fact, a comparison of FIGS. 8(b) and 8(c) with FIG. 3(a) reveals that the out-of-band attenuation is deteriorated by about 20 dB when the impedance matching circuit is provided.
Also on use of the filter, such components as L, C, and R are necessary to provide impedance matching. This undesirably results in increase of the number of parts and fine adjustment is necessary at each impedance matching point. These unfavorable things make the filter difficult to handle and utilize.
In the high frequency region, the out-of-band attenuation is largely affected by a stray capacitance component around a printed circuit board and the location of an inductance, capacitor and the like for impedance matching. The resulting out-of-band characteristics are unstable for practical use.